Remove a commented-out check for headers['Content-Type'].

Remove unused concat method.
Parse the response_headers from the response that PhantomJS receives
Make it possible to control which filename should be used for the downloaded PDF file, by having the
HTML resource respond with a X-Pdf-Filename header containing the desired file name.

(This is only possible for the SynchronousMiddleware.  In the asynchronous Middleware, the code that
sends the PDF file to the user doesn't have access to the Phantom object, since that object was
created in the separate forked process.)